[PATCH 32/39] mm/madvise: use predicates for madvise(..., MADV_DOFORK)

Make it clear what we're blocking in MADV_DOFORK. Previously we simply
disallowed VM_SPECIAL i.e. kernel-owned mappings, fixed mappings and
VMA_IO_BIT.

Now the invariant is established that only kernel-owned mappings can set
VMA_IO_BIT, the VMA_IO_BIT check is redundant.

The rest is equivalent to testing for a kernel-owned or fixed mapping,
i.e. exactly the same check as whether the VMA is permitted to be merged.

This was established by commit 0b2758f48f22 ("Require (reasonably) normal
mappings for MADV_DOFORK") containing my hands-down favourite call out of
all time.

Express the same thing differently - if we wouldn't be allowed to merge it,
then we aren't allowed to manipulate CoW behaviour on fork.

No functional change intended.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/madvise.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 0fc31835fee1..f805a4876c87 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -1400,7 +1400,7 @@ static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)
 		new_flags |= VM_DONTCOPY;
 		break;
 	case MADV_DOFORK:
-		if (new_flags & VM_SPECIAL)
+		if (!vma_can_merge(vma))
 			return -EINVAL;
 		new_flags &= ~VM_DONTCOPY;
 		break;

-- 
2.55.0